Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2001-09-26
2004-09-28
Bell, Bruce F. (Department: 1746)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S006000
Reexamination Certificate
active
06797423
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a high-temperature fuel cell with a fuel-gas chamber disposed between a bipolar plate and an anode of an electrolyte/electrode unit.
It is known that when water is electrolyzed the electrical current breaks down the water molecules to hydrogen (H
2
) and oxygen (O
2
) A fuel cell reverses this procedure. Electrochemical combination of hydrogen (H
2
) and oxygen (O
2
) to give water is a very effective generator of electric current. This occurs without any emission of pollutants or carbon dioxide if the fuel gas used is pure hydrogen (H
2
). Even with an industrial fuel gas, such as natural gas or coal gas, and with air (which may also have been enriched with oxygen (O
2
)) instead of pure oxygen (O
2
), a fuel cell produces markedly lower levels of pollutants and less carbon dioxide than other energy generators in which the energy is introduced from different sources. The fuel cell principle has been implemented industrially in various ways, and indeed with various types of electrolyte and with operating temperatures of from 80° C. to 1000° C.
Depending on their operating temperature, fuel cells are divided into low-, medium-, and high-temperature fuel cells, and these in turn have a variety of technical configurations.
In the case of a high-temperature fuel cell stack composed of a large number of high-temperature fuel cells, there is an upper bipolar plate, which covers the high-temperature fuel cell stack, and under the plate there are, in order, at least one contact layer, an electrolyte/electrode unit, a further contact layer, a further bipolar plate, etc.
The electrolyte/electrode unit here contains two electrodes—an anode and a cathode—and a solid electrolyte configured as a membrane disposed between the anode and the cathode. Each electrolyte/electrode unit here situated between two adjacent bipolar plates forms, with the contact layers situated immediately adjacent to the electrolyte/electrode unit on both sides, a high-temperature fuel cell, which also includes those sides of each of the two bipolar plates which are situated on the contact layers. This type of fuel cell, and others types, are known from the reference titled “Fuel Cell Handbook” by A. J. Appleby and F. R. Foulkes, 1989, pp. 440-454, for example.
A single high-temperature fuel cell provides an operating voltage of less than one volt. A high-temperature fuel cell stack is composed of a large number of high-temperature fuel cells. The connection in series of a large number of adjacent high-temperature fuel cells can give an operating voltage of some hundreds of volts from a fuel-cell system. Since the current provided by a high-temperature fuel cell is high—up to 1,000 amperes in the case of large high-temperature fuel cells—the electrical connection between the individual cells should preferably be one that gives rise to particularly low series electrical resistance under the above-mentioned conditions.
The electrical connection between two high-temperature fuel cells is provided by a bipolar plate, via which the anode of one high-temperature fuel cell is connected to the cathode of the other high-temperature fuel cell. The bipolar plate therefore has an electrical connection to the anode of one high-temperature fuel cell and to the cathode of the other high-temperature fuel cell. The electrical connection between the anode and the bipolar plate is provided by an electrical conductor, which may take the form of a nickel grid (see, for example, German Patent DE 196 49 457 C1). It has been found that the series electrical resistance between the electrical conductor and the bipolar plate is high. This has a serious adverse effect on the electrical performance of the high-temperature fuel cell stack.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a high-temperature fuel cell which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which avoids any relatively high series electrical resistance even when used at high temperatures and to ensure a high conductivity even over prolonged periods.
With the foregoing and other objects in view there is provided, in accordance with the invention, a high-temperature fuel cell. The fuel cell contains a bipolar plate, an electrolyte/electrode unit having an anode, and a fuel-gas chamber formed between the bipolar plate and the anode. An oxidation buffer containing iron is disposed in the fuel-gas chamber.
The object is achieved by a high-temperature fuel cell of the type described in the introduction in which, according to the invention, the oxidation buffer is disposed in the fuel-gas chamber.
Experiments using a high-temperature fuel cell stack have shown that the electrical resistance between the electrical conductor and a bipolar plate formed Cr5Fe1Y
2
O
3
rises, even after a short operating time at normal operating temperatures of between 850° C. and 950° C. The designation Cr5Fe1Y
2
O
3
represents a chromium alloy that contains 5% by weight of Fe and 1% by weight of Y
2
O
3
. The increase in the electrical resistance is caused by an oxide layer which contains chromium oxide and even after a short operating period forms on the surface of that side of the bipolar plate which faces that chamber of the high-temperature fuel cell which carries fuel gas, known as the fuel-gas chamber for short. It also forms where the electrical conductor, for example the nickel grid, rests on the bipolar plate or is connected to the bipolar plate, for example by a weld spot or a soldering point. If the electrical conductor, for example the nickel grid, is spot-welded to the bipolar plate, during operation the oxide, amazingly, even migrates under these contact points, which are in the form of weld spots. Chromium oxide has higher electrical resistance than the unoxidized metals of the bipolar plate. Therefore, there is an oxide layer of poor conductivity between the electrical conductor and the bipolar plate, and this has an adverse effect on the series resistance of series-connected high-temperature fuel cells. The formation of chromium oxide takes place even at oxygen partial pressures of less than 10
−18
bar. These oxygen partial pressures are generally also present in the fuel-gas chamber while the high-temperature fuel cell is operating.
The invention is based on the idea that a relatively high series electrical resistance is avoided and a high conductivity is ensured even over relatively long periods if the formation of the oxide layer on the bipolar plate is suppressed. This is achieved by the fact that the oxygen that is situated in the fuel-gas chamber of the high-temperature fuel cell while the latter is operating is taken up and stored by the oxidation buffer. Consequently, oxygen is withdrawn from the fuel gas and is then no longer available for oxidation of the bipolar plate. For this purpose, the oxidation buffer is configured in such a way that it takes up oxygen from the environment.
The term oxidation buffer is understood as meaning a device that takes up oxygen from the fuel gas during a with-load phase operation of the high-temperature fuel cell and releases the oxygen again during an idling phase of operation. In this way, the oxidation buffer is regenerated for a further with-load operating phase. The term with-load operation refers to the operating mode of the high-temperature fuel cell in which the high-temperature fuel cell generates significant amounts of electric current by the electrochemical combination of fuel gas and oxygen. The electrochemical combination does not take place during an idling mode. The idling mode is introduced, for example, by disconnecting the high-temperature fuel cells from a consumer.
In this context, an oxygen storage that takes up oxygen but cannot be regenerated again while the high-temperature fuel cell is operating is not considered to be an oxidation buffer. The property of being capable of regeneration, i.e. the possibility of removing the stored oxygen from the oxidation
Fleck Robert
Kaiser Wolfram
Bell Bruce F.
Greenberg Laurence A.
Locher Ralph E.
Siemens Aktiengesellschaft
Stemer Werner H.
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